In wireless communication systems, base stations are traditionally equipped with a small number of antennas. A radically different approach involves base stations with an unprecedented numbers of antennas (M) simultaneously serving a much smaller number of mobile terminals (K where M>>K) using multi-user beamforming. Operating with a large ratio of antennas to terminals under simultaneous service can yield large increases in both spectral efficiency and energy efficiency. As the number of service antennas increases and power is reduced the most simple signal processing, conjugate beamforming on the forward link and matched-filtering on the reverse link, asymptotically achieves near-optimal performance. One technical challenge is rapid channel state estimation.
With M base station antennas and K terminals, a large antenna array (LSAS) base station deals with 2M×K channels. For each terminal, the base station can get all M uplink channels with one pilot transmission from the terminal. However, for the M downlink channels, the base station may have to send M pilots. In addition, the terminal will need to feed back the channel estimations to the base station. Accordingly, a total of (M+1)K pilot transmissions would be needed. Scaling is therefore a problem in this system.